Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change the mechanical compression ratio and a variable valve timing mechanism able to control the closing timing of the intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and at the time of engine high load operation, and increasing the mechanical compression ratio and retarding the closing timing of the intake valve as the engine load becomes lower in the state holding the actual compression ratio constant at the time of engine medium and high load operation (for example, see Japanese Patent Publication (A) No. 2004-218522).
In this regard, in the internal combustion engine disclosed in Japanese Patent Publication (A) No. 2004-218522, at the time of engine low load operation, the mechanical compression ratio is made larger, the closing timing of the intake valve is retarded, and further no supercharging is performed. When retarding the closing timing of the intake valve in the engine low load operation region in this way, to make the intake air amount supplied into the combustion chamber the target intake air amount, it is necessary to make the opening degree of the throttle valve larger to a certain extent. In this way, if the opening degree of the throttle valve is large at the time of engine low load operation, the negative pressure generated in the engine intake passage at a downstream side of the throttle valve (for example, surge tank) is not that large.
On the other hand, for example, a brake booster, a canister for purge control, and other devices utilizing negative pressure (hereinafter referred to as “negative pressure utilizing devices”) usually are driven utilizing the negative pressure generated in an engine intake passage. For this reason, if sufficient negative pressure is not generated inside the engine intake passage at the time of engine low load operation as explained above, it is no longer possible to suitably operate these negative pressure utilizing devices. In particular, a brake booster requires a relatively large negative pressure to generate sufficient braking force, so generation of a large negative pressure in the engine intake passage becomes necessary.
In this regard, as explained above, in the internal combustion engine disclosed in Japanese Patent Publication (A) No. 2004-218522, it is not possible to generate a large negative pressure in the engine intake passage at least at the time of engine low load operation and therefore it is not possible to suitably operate negative pressure utilizing devices.